A gas sample can be made up of a plurality of atoms, molecules, and chemical compounds. In the context of a mass-spectrometric detection, ionization of a sample is accomplished via photon and/or electron irradiation; depending on the nature and intensity of the irradiation, a selective ionization of the various atoms, molecules, or chemical compounds, or a fragmentation of molecules and compounds, can take place. The ions that are generated are deflected by an electric field and conveyed to a mass-spectrometric detection system.
The resonance-enhanced multi-photon ionization technique (REMPI), which utilizes UV laser pulses (soft photoionization) for selective ionization of, for example, aromatic compounds, is used as a soft and selective ionization method for mass spectrometry. The selectivity is determined by, among other factors, the soft UV spectroscopic properties and the location of the ionization potentials. The REMPI method is disadvantageous in that it is limited to certain substance classes, and the ionization cross section can in some cases be extremely different even for similar compounds.
Single photon ionization (SPI) using VUV laser light likewise permits partially selective and soft ionization. Selectivity is determined by the location of the ionization potentials. A typical application is the detection of compounds that cannot be detected using REMPI. The SPI method is disadvantageous in that here as well, some substance classes cannot be detected. In addition, selectivity is lower than with the REMPI method, so that greater interference can occur with complex samples.
On the other hand, the unselective but fragmenting electron impact (EI) ionization method using an electron beam is a standard technique in mass spectrometry for ionization in particular of volatile inorganic and organic compounds. It acts on all substances (i.e. not selectively), and with many molecules often results in extreme fragmentation. It is particularly suitable for detecting compounds (such as e.g. O2, N2, CO2, SO2, CO, C2H2) that are difficult to sense by photon ionization as mentioned above using UV and VUV radiation (SPI, REMPI).
When a gas sample having a plurality of compounds is ionized using the SPI method, however, it can happen that multiple compounds having the same mass are ionized, and therefore cannot resolved mass-spectrometrically. With EI ionization of a gas sample having a plurality of compounds, it can happen that multiple compounds having the same mass and/or a similar fragmentation pattern are ionized, and here as well individual compounds cannot be resolved. It is useful in this respect to direct the gas sample through a gas chromatograph (GC) capillary for preselection of the compounds, so as thereby to achieve in the gas flow a time offset, which can be traced back and thus allocated to the individual compounds, between the compounds before admission into the ionization chamber.
Proceeding from the aforementioned types of irradiation, DE 100 14 847 A1 describes a technology for detecting compounds from a gas flow, which technology utilizes a combination of the aforesaid SPI and REMPI ionization. Alternating irradiation of a continuous gas flow with REMPI and SPI ionization pulses (UV and VUV laser pulses, respectively) is performed in this context, a separate isolated volume element being ionized with each pulse and conveyed to a mass spectrometer. All the laser pulses are generated with the aid of a configuration having solid-state lasers and having a plurality of optical elements that are in part also modifiable.
With the aforementioned technology, however, only selective types of radiation are used, so that certain substances that are ionizable only by an electron beam are not sensed. In addition, the ions are generated in this case exclusively by laser pulses on the axis of the time-of-flight mass spectrometer. Continuous ion sources cannot be used here.
The solid-state lasers used to generate UV or VUV irradiation also have only a very limited repetition rate in the region of 50 Hz. If the compounds of a gas flow are first preselected in a GC capillary, however, changes in the gas-flow composition (typically with very brief concentration peaks) may be expected; this requires an enhanced time resolution and redundant measurements in rapid sequence. A repetition rate of the aforesaid magnitude is no longer sufficient, however, and results in incorrect measurements.
In addition, ordinary (i.e. non-tunable) solid-state lasers generate only one wavelength, which necessitates the aforementioned complex configuration having a number of optical elements.